The present invention relates, generally, to a network or system for transmitting both data and television signals between a plurality of digital computer ("PC") workstations and/or "stand-alone" stations. More particularly, the invention relates to two-way multimedia communications in a local area network (LAN) environment.
Multimedia communications concern the transfer of digital data along with video and/or audio information. Multimedia applications software is available that permits digital computer workstations, particularly desktop personal computers or "PCs", to access multimedia program sources. One-way multimedia communications are particularly well-known in instructional programs.
Live two-way multimedia communications, generally referred to as "videoconferencing", have heretofore required a major investment, involving a dedicated room, expensive single-purpose hardware and specially-trained operators. Less ponderous conferencing hardware, conferencing hardware that is more user-friendly, preferably including multiple-use components, is needed.
As multimedia applications are incorporated into digital computer ("PC") workstations it becomes possible to use such workstations for videoconferencing with the users of one or more similar workstations at either nearby or remote locations. To achieve this capability, such a workstation must include, as a minimum:
(1) an image display (e.g., CRT display); PA1 (2) a sound transducer (e.g., loudspeaker); PA1 (3) a digital computer for processing data, connected to supply image and control signals to the digital display for imaging data; PA1 (4) a circuit for converting the video portion of a television signal into image and control signals compatible with the image display for displaying a picture on this imaging device; and PA1 (5) an analog amplifier or the like which provides the proper gain and impedance for driving the sound transducer with the audio portion of the TV signal.
A conversion circuit of the aforementioned type, which digitizes an analog video signal and formats this digitized signal in such a way as to permit display of the video image on a digital computer display, is well known. Such a circuit, called a "video window controller", is available for both the DOS-based Personal Computer and the PS/2 computer of International Business Machines Corp., as well as for the Macintosh computers of Apple Computer Corp. This circuit is used to view standard recorded video, such as VCR program material, or to view live images from a standard video camera, on the monitor screens of the workstations.
If two-way communication with a digital computer workstation is desired, it is necessary also to provide the workstation with a video camera, aimed appropriately at the workstation user, for producing a video signal representing the image of the workstation user, and a microphone arranged to pick up the voice of the workstation user and to generate an analog audio signal in response thereto.
If computer circuitry for controlling an image display is not present, a display can be driven by another circuit, called a "video display processor", which converts video signals into the standard or protocol required to operate the display. Such circuits are conventionally employed in the art of video games.
The term "information signal", as used herein, is intended to mean any signal, analog or digital, which conveys information such as data or graphics. An "information signal" may thus include a "data signal" and/or a "television signal", depending upon the type of information transmitted, and/or some other kind of information signal.
The term "television signal" or "TV signal", as used herein, is intended to mean a conventional (NTSC or other standard) analog signal, which includes both a video and an audio portion, and/or any other standard or non-standard representation of video and/or audio information including digitally encoded information (compressed or uncompressed). The terms "video signal" and "audio signal" will be used to separately denote only the video portion and audio portion, respectively, of the television signal. As is well known, for NTSC standards, the video portion lies within a frequency range of 0 to 4.75 MHz whereas the audio portion lies within a frequency band of 0 to 15 KHz. As desired, these video and audio signals are typically combined and modulated upward from baseband to a 6 MHz wide (in Europe, a 7 MHz wide) frequency channel within a broadband spectrum of 30 to 800 MHz.
Similarly, broadband (30-800 MHz) television signals containing one or more active frequency channels can be tuned and demodulated to produce the video and audio portions (signals) of a single television signal at baseband.
As used herein, the term "baseband signals" is intended to define information signals within the frequency range of 0 to 30 MHz, or perhaps 0 to 50 MHz. Computer data is normally transmitted over a network by baseband signals. The term "broadband signals" is thus intended to mean information signals at a frequency higher than the highest baseband frequency--typically in the frequency range of 40 to 600 MHz. If the upper limit of the baseband range terminates at a lower frequency, for example, 25 MHz, the lower limit of the broadband range can commence at this lower frequency. The highest broadband frequency is determined only by the limitations of the transmission technology (e.g. CATV cable) and the applicable government standards, if any.
It is known in the television industry to transmit the audio portion of a television signal--i.e., the "audio signal"--with frequency modulation (FM) on a carrier at 41/2 MHz. Among television industry standards, only the French standard SECAM transmits sound with amplitude modulation (AM). The video portion of the television signal ("video signal") with most TV standards is transmitted in AM with a given polarity.
It is known to transmit both baseband and broadband signals on a common "backbone communication network" such as a local area network (LAN) which is connected to a plurality of user workstations. The U.S. Pat. No. 4,885,747 to Foglia discloses a so-called "filter coupler" or "F-coupler" by which (1) baseband signals (data) are transmitted between a backbone network and a given workstation via a twisted pair shielded cable in a balanced mode, and (2) broadband (television) signals are transmitted between the same backbone network and the same workstation via the same shielded cable in an unbalanced mode. The disclosure of this U.S. patent is incorporated herein by reference. A coupler which enables the transmission of both baseband and broadband signals on a LAN twisted pair cable is termed a "video coupler" or "V-coupler"
With the Foglia system, a number of television programs (satellite television, VCR or a "live" broadcast from a television camera) is "broadcast" to all workstations connected to the LAN (an IBM Token Ring, in this case) from a coaxial video cable through a so-called "tap/combiner". While each workstation can select one from a number of television channels that are broadcast on the broadband frequencies, and while it is even possible for a workstation to broadcast to all other workstations by means of a TV camera or some other program source, it is not possible for any particular workstation to transmit television signals to any other particular workstation or workstations on the LAN, or to transmit to any remote workstation not connected to the LAN, thereby to provide true videoconferencing capability.
The U.S. Pat. No. 4,564,940 to Yahata discloses a so-called "broadband network system" which includes a private branch exchange (PBX) for interconnecting a plurality of workstations. However, this system is intended to replace an industry standard local area network (LAN) for the multiplex communication of voice and data. No consideration is given to the special problems encountered by the transmission of television signals.
The U.S. Pat. No. 4,675,866 to Takumi et al. discloses a transmission system between workstations that provides both a baseband and broadband capability. One or more channels in the broadband network are used for effecting transmission of signals of the baseband network. A central re-transmission facility serves to provide videotext to a plurality of workstations. No videoconferencing capability is contemplated or disclosed.
The U.S. Pat. No. 4,814,869 to Oliver, Jr. discloses a "video surveillance system" in which modulated signals from a plurality of video cameras are multiplexed onto a single path capable of carrying, for example, up to 36 video channels. One or more such communication paths are provided to a signal splitter which provides the paths to one or more video screens and tuners. The tuners are operated under control of a computer so as to sequence the display of information from the different video cameras onto the video screens.
The U.S. Pat. No. 4,977,449 to Morgan discloses a similar video surveillance system in which both the modulators at the video cameras and the demodulators at the monitors are "frequency agile". In this case, the control computer controls the channel selection of both the modulators and the demodulators. In both the Oliver Jr. and Morgan patents, however, no videoconferencing capability is contemplated or disclosed.
The U.S. Pat. No. 4,935,924 to Baxter discloses a signal distribution cable network in which information signals from different signal sources, such as cable television, FM radio, videotape recorder, video camera and compact disk player, are transmitted on a common cable at different frequency channels. A single channel allocation "controller" is connected on the cable and transmits channel selection signals on the cable to both the information sources and the information users (receivers) to control the channel allocation. Again, no videoconferencing capability is contemplated or disclosed.
The U.S. Pat. Nos. 4,686,698; 4,710,917; 4,716,585; 4,847,829 and 5,014,267 of Tompkins et al. disclose a videoconferencing network in which a plurality of video terminals are connected in a star configuration to a "MIX" switching network. The MIX switching network operates in the manner of a "telephone switch" to connect one of the video terminals to one or more of the other video terminals. Each cable connecting a video terminal to the MIX carries baseband frequencies and two channels above baseband for the two-way transmission of audio and video information. MIX switches can be interconnected but terminals can only be interconnected via one or more MIX switches.
The French Patent No. 2,590,429 discloses a video terminal having both a camera and an image display. Broadband television signals are received on one cable and transmitted on another, in digital form. Time division multiplexing is used to transmit plural signals. As in Tompkins et al., the video terminals are arranged in a star configuration.
The article "Video-und Datenkommunikation im VBN" by Pernsteiner et al., NTZ Nachrichtentechnische Zeitschrift (Vol. 42, No. 8, August 1989) pp. 486-493 discloses a "video workstation" (that is, a PC workstation adapted to transmit and receive both video and audio) which is connected to a glass fiber communication network established by the German Telephone and Postal Authority (Bundespost) known as the "Preliminary Broadband Network" (VBN). It is contemplated that each video workstation be either coupled directly to the VBN, or coupled thereto via a "mega-switch". In the latter instance, the video workstations are connected to the mega-switch in a star configuration.
The European Patent Publication No. 0,119,588 discloses an "integrated information system" in which a videoconferencing station (so-called "BIGFON Island") sends digitized video, audio and data signals via an optical fiber transmission cable to a video/telephone switching network. All of the video stations are connected to this telephone switch in a star configuration.
Similarly, the article "Software Architecture for Integration of Video Services in the Etherphone System" by Rangan et al., IEEE Journal on Selected Areas in Communication, Vol. 9, No. 9, December 1991, pp. 1395-1404 discloses a videoconferencing system having a plurality of videoconferencing workstations all connected in a star configuration to a matrix switch. The matrix switch is controlled by a computer called a "central connection manager".
All of these known videoconferencing systems are relatively complex, require a dedicated computer for controlling the interconnection of video workstations and/or the selection of transmission channels on a multichannel cable. As a consequence, these systems are relatively expensive to implement on a per/workstation basis.